Structure and phase transformation of nanocrystalline and amorphous alloy thin films.

摘要

Structure and phase transformation of amorphous and/or nanocrystalline alloys are studied systematically using in-situ electron diffraction (ED) and imaging (high-resolution transmission electron microscopy (HR-TEM) and high-angle annual dark-field scanning transmission electron microscopy (HAADF-STEM)). Nanocrystalline Ag-Cu and Ag-Si thin films and amorphous Cu-Zr thin films of different compositions and thicknesses are synthesized by sputtering deposition and characterized by the morphology and atomic structures.; The observations of morphology and structure of AgxCu 1-x thin films show two stages of transformation. The first stage is associated with decomposition from the solid solution phase to two terminal phases (Ag- and Cu-rich), while the second stage is associated with film dewetting and the formation of crystal grains of 101 nm. The first stage transition corresponds to the spinodal decomposition observed earlier in amorphous Ag-Cu alloys. The structure analysis by electron diffraction shows that the initial stage of decomposition is associated with the growth of Cu crystallites in Ag50Cu50 and Ag-rich films and the initial phase separation is also accompanied by strain.; The systematic study on the structure and morphology of Ag-Si alloy thin films shows nanocrystalline Ag clusters embedded in amorphous Si matrix for samples of different compositions and thicknesses. Ag cluster sizes increase from 2.5 nm to 3.5 nm as annealing temperature increases from room temperature to 500°C, and the cluster densities decrease accordingly from 2.5x10 4/mum2 to 7.6x103/mum 2. This phenomenon can be explained by grain growth and Ostwald ripening in which big clusters grows bigger at the expense of smaller clusters. The Ag clusters remain relatively stable under annealing; HREM images show crystalline structure at 500°C, which is different from Ag clusters supported on other substrates. TEM observation shows that Ag clusters are embedded inside the amorphous Si matrix, which explains their relative stability at high temperatures.